7 Answers
7

ISO is very useful as it helps overcome read noise by amplifying a weak analogue signal prior to digitization (which adds a more or less constant amount of noise) thus giving a better signal to noise ratio.

That's all raising the ISO does, amplify the signal. It does not make the picture noisier because it only amplifies what's already there.

See this example. The ISO100 shot was significantly underexposed and suffers from really bad read noise. The same amount of light enters the camera in the second shot, but due to ISO the signal is amplified before readout, thus read noise is a smaller percentage of the signal and overall signal to noise ratio is better:

To using a high ISO actually helps to reduce noise when you have a limited amount of light (and you can't get any more by opening the aperture or shutter for longer).

Good write-up! But you should include in your post that the exposure of the ISO 100 shot was corrected in post.
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eWolfMay 28 '11 at 4:20

My understanding is that of the 2 RAW images before any processing is done the ISO1600 image will have more noise. Yes, both images have had the same steps done on them in post, but the auto contrast function will have performed more gain increase on the ISO 100 image meaning more than one variable has changed so the experiment isn't valid.
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PhilMay 29 '11 at 14:04

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But the brightness of the overall image is part of the output, which you expect to change as you alter the input variables, with all intermediate steps remaining constant.
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PhilJun 13 '11 at 0:19

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I think I've finally worked out how the experiment works. The single variable this is changing is the position of the amplification in the overall signal path, which is affected by the ISO setting. In the ISO1600 example the majority of the amplification is earlier in the overall processing chain. Whereas when using ISO 100 the majority is later on. The amplification provided by the iso setting + post processing does remain constant. So the conclusion is the earlier in the signal chain the amplification is provided the better, and this is achieved by using a higher ISO.
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PhilJun 13 '11 at 1:59

Yes, it is useful as there are some situations where there just isn't enough light to take photos at low ISOs. I take quite a few photos of the am-dram productions I'm involved in, and even with a F1.8 lens I often find myself shooting at an ISO between about 800 and 1600 in order to be able to have a short enough shutter speed to freeze the action. It's not the extreme of 64,000 that you've mentioned but it's certainly a higher ISO than I use if I'm shooting outside during the day time.

Shooting action in low-light conditions or freezing very fast action are common situations where you might need to compromise on the ISO to get the shot.

For example, I recently shot some pictures of jets passing by at an air show. I found that even at noon on a day with clear skies and the aperture at 2.8, ISO 100 was too low to get me the shutter times I wanted. I had to go to ISO 800 to get 1/8000 shutter times. Now, ISO 800 produces perfectly good results with my camera, but if I could have shot even faster shutter times than 1/8000 then I would definitely have tried out ISO 1600 too, as even 1/8000 wasn't always fast enough to freeze an F-15 buzzing the crowd.

Regarding high ISO for effects, I rarely find chroma noise pleasant to look at so I usually post-process high ISO pictures in black and white. While luminance noise from a digital sensor doesn't compare to the grain of high ISO black and white film, it sometimes provides passable results with a little post-processing.

Here's an example of a night time shot with ISO 6400. It's not a technically very good shot--the noise is very pronounced--but it's a shot I couldn't get any other way as I didn't bring any flashes with me that day.

To answer the second question you ask, ISO really does affect the performance and one thing to keep in mind is that the higher the ISO you use the more noise you will end up with in your image. Although, this is being improved upon and becomes less of an issue with each successive generation of cameras.

I'd say it's extremely useful, because photography is about capturing good looking light, and it's easier for light to look good when there's not much of it, e.g. at night, at sunset/dawn, in pubs, at concerts, in the woods.

That's why the industry strives to make sensors (and film, in the past) with good high iso performance and fast lenses, and photographers are willing to buy the equipment that is most advanced with regard to these aspects.

it depends on a sort of photography. Sometimes bad quality photo is better than no photo :-)
It also depends on type of sensor. My Exmor CMOS chip on my Pentax K-x has very good quality so I can use ISO 800 for very good photos, ISO 1600 for web and small sized and ISO 6400 when really necessary and when you do not have much alternatives how to take the photo. My older camera had so bad CCD chip, that even ISO 800 was bad and ISO1600 was really terrible.